Molecular crystals based on hybrid organic perovskites (HOP) of general formula (R-NH3)pMXn, where R is an organic group, M a divalent metal and X a halogen, are a new class of semiconductors exhibiting fabulous optical properties. In particular, the HOPs of formula (R-NH3)2PbX4 present a multi-layered structure, composed of PbX62- inorganic layers alternating with organic layers, whose electronic structure is the one of a multi-quantum well. Due to strong quantum and dielectric confinements, excitonic effects very stable at room temperature and oscillator strengths one order of magnitude larger than in GaAs/(Ga,Al)As heterostructures are found, making these molecules very attractive for light-emitting devices such as electroluminescent diodes and lasers. Since 2012, the HOP of formula CH3NH3PbI3 represents a “material breakthrough” for photovoltaics : in only 3 years, the efficiency of CH3NH3PbI3-based solar cells has progressed from 12% to 21%, competing with the efficiency of the silicon-based cells ! Additionally to the interesting optical properties, an impressive property of these materials is related to amazingly good transport properties for both electrons and holes, opening the way to electrically injected opto-electronic devices based on molecular crystals, carried out with soft chemistry processes.This presentation will focus on the optical properties of HOPs. We will explain why these materials can be considered as a new class of semiconductors and show in particular that HOPs combine strengths of the inorganic semiconductors and organic semiconductors, likely to solve the contradiction that high charge carrier mobility and large stimulated emissions are required for lasing devices but cannot be found, in general, in the same material.